1. Field of the Invention
The present invention relates to an internal combustion engine controller having a function of performing atmospheric learning in order to calibrate an oxygen concentration sensor detecting an oxygen concentration in an exhaust gas of an internal combustion engine.
2. Description of Related Art
Recent computerized automobiles are configured to control an air-fuel ratio in order to increase the cleaning factor of an exhaust gas cleaning catalyst on the basis of the output value of an oxygen concentration sensor installed in an exhaust passage.
The oxygen concentration sensor has a problem in that its sensing accuracy varies depending on manufacturing variation (individual difference), and deteriorates with the passage of time. Accordingly, it is common to perform atmospheric learning in which the oxygen concentration sensor is calibrated based on the assumption that the space around the oxygen concentration sensor installed in the exhaust passage is filled with the atmospheric air after the lapse of a predetermined wait time from a time when the fuel supply to the internal combustion engine is cut off, and the output value of the oxygen concentration sensor therefore indicates the atmospheric oxygen concentration.
It should be noted that a combusted gas remains in the upstream of the oxygen concentration sensor immediately after the fuel cutoff, and accordingly the oxygen concentration around the oxygen concentration sensor does not approach to the atmospheric oxygen concentration until the combusted gas is replaced by new air (atmospheric air). The time needed for the oxygen concentration around the oxygen concentration sensor to become substantially equal to the atmospheric oxygen concentration (referred to as delay time hereinafter) from the time of the start of the fuel cutoff depends on a running state of a vehicle on which the internal combustion is mounted. Accordingly, it is known to change the above described wait time depending on the engine rotational speed, vehicle speed, or gear shift position immediately before the time of the start of the fuel cutoff, as disclosed, for example, in Japanese Patent Publication No. 2003-3903.
However, the factors that affect the above described delay time are not limited to the engine speed, vehicle speed, and gear shift position immediately before the time of the start of the fuel cutoff. For example, in an internal combustion engine configured to return its exhaust gas from an exhaust passage to an air intake passage thereof, the delay time becomes long as the returning amount of the exhaust gas increases causing the amount of intake air to decrease. Accordingly, it is difficult to set the wait time to an optimum value in the conventional internal combustion engine controllers configured to change the wait time depending on the engine speed, vehicle speed, and gear shift position immediately before the time of the start of the fuel cutoff.
If the wait time is set shorter than the delay time, the atmospheric learning may be erroneously performed before the oxygen concentration around the oxygen concentration sensor becomes substantially equal to the atmospheric oxygen concentration. On the other hand, if the wait time is set longer than the delay time, the atmospheric learning may not be performed with a sufficiently high frequency.